In some prior art techniques for manufacturing integrated circuits, it is desired to transform the apparent silicon surfaces into a refractory metal silicide; for example, in case of MOS transistors, it may be desirable to carry out this operation on the surface of the polycrystalline silicon gate areas and on the surfaces of the drain and source areas which are doped areas in a monocrystalline silicon substrate.
A conventional process for realizing this operation will be described herebelow in relation with FIGS. 1-4 in case the refractory metal is tungsten.
FIG. 1 shows a schematical section view of a MOS transistor structure at an intermediate manufacturing stage. The structure of FIG. 1 is formed in a silicon subtrate 1, in an area delineated by thick oxide regions 2.
The MOS transistor comprises a polycrystalline silicon gate 3 formed on a gate oxide layer 4. The gate edges are isolated by an oxide layer usually called a spacer 5. The drain 6 and the source 7 exhibit in FIG. 1 a stepped structure because they are formed in two steps; a first step in which the thick oxide region and the gate itself serve as a mask and a second step in which it is the gate, enlarged by the spacers, that serves as a mask.
If, at this manufacturing stage, a tungsten silicide layer is to be formed at the surface of the gate, and at the surface of the source and drain, a tungsten layer 10 is uniformly deposited (FIG. 2) for example through cathodic sputtering. A thermal process ranging from 700.degree. C. to 1000.degree. C. is then carried out; as a result, tungsten reacts with the apparent silicon surfaces for forming a tungsten silicide WSi.sub.2, referenced 11 in FIG. 3. This thermal process is made under vacuum conditions or in an atmosphere neutral with respect to tungsten.
Following that step, as shown by FIG. 4, the tungsten layer is eliminated by means of selective etching.
This conventional process is advantageous in that only simple steps are involved, namely a uniform metal deposition, and no lithographic step is necessary for positioning tungsten silicide.
However, this process is inefficient in case of very small size structures (minimum dimension lower than one micrometer). Indeed, the extension, and more particularly the lateral extension, of the WSi.sub.2 regions depends upon the annealing time that has to be controlled with the greatest accuracy. Moreover, the presence of small native oxide areas on the theoretically bared silicon areas causes an inhomogeneous thickness of the tungsten silicide layer, since tungsten does not react with SiO.sub.2, and, if the reaction is continued for palliating this difficulty, the lateral extension of the tungsten silicide is unduly increased.
The same problems are encountered with molybdenum. They have been partially solved as far as titanium is concerned by causing titanium to react with nitrogen under atmospheric pressure conditions during the annealing process. This process leads to an equilibrium stage between the formation of titanium nitride and titanium silicide and therefore permits to obtain a well defined reaction final state. However, this process has not been envisaged for molybdenum and tungsten which do not react with N.sub.2 at the annealing temperatures. Moreover, in case of tungsten, the process, implemented under ambiant pressure conditions, supplies too thin silicide layers.